1. Field of the Invention
This invention relates to an organic electroluminescent device having excellent light-emitting properties.
2. Description of the Related Art
An organic electroluminescent device (which will hereinafter be called "organic EL device" ) is a light-emitting device which makes use of the principle that when an electric field is applied, a fluorescent material emits light in response to the charge recombination of holes injected from an anode and electrons injected from a cathode. After C. W. Tang et al. of Eastman Kodak Company reported a low-voltage-driven organic EL device using a double layered structure (C. W. Tang, S. A. Vanslyke, Applied Physics Letters, 51, 913(1987) and the like), studies on an organic EL device have been briskly carried out. Tang et al. reported an organic EL device using tris(8-hydroxyquinolinol aluminum) in a light-emitting layer and a triphenyldiamine derivative to a hole-transporting layer. This stacked structure gives such advantages as an improvement in the injection efficiency of holes into the light-emitting layer; blocking of electrons injected from a cathode, which increase the efficiency of exciton production from charge recombination; and confinement of the excitons into the light-emitting layer. A double layered structure composed of a hole-injecting and transporting layer and an electron-transporting and light-emitting layer or a triple layered structure composed of a hole-injecting and transporting layer, a light-emitting layer and an electron-injecting and transporting layer is well known as an organic EL device. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such multi-layered devices. As a hole-transporting material, triphenylamine derivatives such as 4,4',4"-tris(3-methylphenylphenylamino)triphenylamine and aromatic diamine derivatives such as N,N'-diphenyl-N,N' bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine, which are starburst molecules, are well known (e.g., Japanese Patent Application Laid-Open Nos. 20771/1996, 40995/1996, 40997/1996, 53397/1996 and 87122/1996).
As an electron-transporting material, oxadiazole derivatives, triazole derivatives and the like are well known. As a light-emitting material, known are chelate complexes such as tris(8-quinolinolate)aluminum complex, coumarin derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives, oxadiazole derivatives and the like. Since various color light in a visible region from blue to red are obtained from these light-emitting materials, there increased expectation for industrialization of a full color organic EL device (e.g., Japanese Patent Application Laid-Open Nos. 239655/1996, 138561/1995, 200289/1991 and the like). In recent days, organic EL devices with high brightness and long lifetime have been disclosed or reported, but any of them are not enough in these properties.